When I was worried about Kalina having milk I took her to the Vet. He tried to express some milk into a teeny syringe and could honestly only get out about a drop or two. But he still said she was fine and had milk and, obviously, he was right. When the puppies nurse the sucking and pushing stimulate the milk. As long as they have round bellies and are gaining, they are probably ok. If you have any doubts, give them some milk by dropper. Sounds like they are doing well. I'm so happy for you.
